1. Field of the Invention
This invention relates to a process for the production of high purity, light color 2-mercaptotoluimidazole and metal salts thereof in high yields.
2. Description of the Prior Art
It is known that 2-mercaptotoluimidazole can be obtained by reaction of o-toluenediamine with carbon disulfide in a solvent. The reaction is accelerated by the addition of catalytic quantities of alkali metal hydroxides or tertiary amines. However, the product thus obtained is discolored and contains impurities which are difficult to remove by subsequent purification processes. The poor quality of the product is caused by the use of discolored o-toluenediamine as a starting material. o-Toluenediamine is highly susceptible to discoloration upon exposure to air, heat, light, moisture and the like. As a result, autooxidation products are formed almost immediately after preparation of the amine. In the preparation of 2-mercaptotoluimidazole, the autooxidation products are coprecipitated as impurities thus causing discoloration and further deterioration of the imidazole. For applications where light color 2-mercaptotoluimidazole is desired, high purity o-toluenediamine must be used as a starting material. This can be achieved by distillation of the amine and immediate use thereafter. However, in practical application, it is not always possible to combine the distillation step with production. Another suggested approach is storage of the amine under an inert gas at positive pressures. However, the treatment is of temporary nature and upon further handling or processing, the stability is easily lost. Special equipment and handling techniques are required which are often inconvenient for large scale operations.
Toluenediamines have been also stabilized by addition of stabilizers. Thus, U.S. Pat. No. 2,894,034 discloses the addition of elemental sulfur. However, in the preparation of 2-mercaptotoluimidazole, sulfur coprecipitates and it is necessary to remove the stabilizer from the final product by recrystallization or other purification methods. It has now been discovered that 2-mercaptotoluimidazole of high purity and light color can be prepared by using a stabilized raw material system without a separate stabilizer removal step.